1. Field of the Invention
The present invention is concerned with the field of quantum communication systems and emitters and receivers which may be used in such systems. Specifically, the present invention is concerned with the use of a reference pulse in a quantum communication system in order to provide active stabilisation of the system.
2. Discussion of Background
In quantum communication systems, information is transmitted between a sender and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarisation, phase or energy/time. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
Quantum key distribution which is a technique for forming a shared cryptographic key between two parties; a sender, often referred to as “Alice”, and a receiver often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper (Eve). In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information.
Examples of quantum communication systems are described in GB 2 368 502 from the current applicant.
When the photons are encoded using phase, typically, a Mach-Zender interferometer is provided in both Alice's sending equipments and Bob's receiving equipment. Each interferometer has a long path and a short path. Details of how the photons are encoded using this arrangement will be described later. However, it is required that photons that contribute to the key or the encoded information through the short arm of one interferometer and the long arm of the other interferometer. Thus, the photons may follow one of two paths: Path 1, the short arm of Alice's interferometer and the long arm of Bob's interferometer; and Path 2, the long arm of Alice's interferometer and the short arm of Bob's interferometer.
Both interferometers will contain a phase modulator which can be used to either randomly vary the phase of photons passing through the interferometer either randomly or under the control of either Alice or Bob.
However, it is necessary that any other phase delay between Path 1 and Path 2 is constant throughout transmission as any other phase delay can increase the quantum bit error rate and can even make the system unusable if it exceeds a certain level. Thus, in practice, one has to calibrate the phase delay every several tens of seconds or several minutes depending on the stability of the system. This introduces a dead time to the system. Also, during key distribution, no information concerning the phase drift can be obtained. This causes extra difficulties in identifying the presence of an eavesdropper as Alice and Bob cannot identify the source of the quantum bit error rate. It can arise from either an eavesdropper or a variation in the phase drift.
It is also required that the polarisation of photons be stabilised. However, this presents difficulties as photons will generally be sent from Alice to Bob along a single mode fibre link and the polarisation of photons passing through this link will vary due to birefringence regions commonly existing in the single mode fibre. For example, variations in the temperature can cause the polarisation to vary randomly. It is necessary to be able to correct any rotation of the polarisation which has occurred in the fibre link because some of the components of Bob's equipment are polarisation sensitive and variations in the polarisation will again result in a higher error rate. Also, the bit rate of the system may decrease in equipment where polarisation beam splitters are used to ensure that photons pass through the short arm of one interferometer and the long arm of the other.